Drugs affecting Cardia Rate and Force

Question 1

What kind of β adrenoceptors are in the heart and the lungs?

Answer 1

β1 adrenoceptors in the heart.

β2 adrenoceptors in the lungs.

Question 2

What is the mechanism by which the sympathetic nervous system affect cardiac rate and force?

Answer 2

Noradrenaline activates β1 adrenoceptors in nodal and myocardial cells. Coupling through Gs protein alpha subunit stimulated adenylyl cyclase to increase the intracellular concentration of CAMP. This causes cellular responses.

Question 3

What are the effects of the sympathetic nervous system on cardiac rate and force?

Answer 3

\+ve Chronotropic effect
\+ve inotropic effect
\+ve dromotropic effect
\+ve lusitropic effect
Increase in automaticity.
Increase in the activity of Na+/K+ATPase.
Decreased mass of cardiac muscle.

Question 4

What is a chronotropic effect?

Answer 4

Changes in heart rate.

Question 5

What is a inotropic effect?

Answer 5

Changes in the force in the heart.

Question 6

What is a dromotropic effect?

Answer 6

Changes in conduction velocity.

Question 7

What is a lusitropic effect?

Answer 7

Changes in the rate of relaxation of the heart.

Question 8

What is automaticity?

Answer 8

This is a tendency for non-nodal regions to acquire spontaneous activity.

Question 9

What is the mechanism by which the parasympathetic nervous system affect cardiac rate and force?

Answer 9

Acetylcholinr activates M2 muscarinic cholinoceptors in nodal. Coupling through Gi protein-
via alpha subunits inhibits adenylyl cyclase reduces cAMP.
via beta/gamma subunit dimer opens specific potassium channels (GIRKs) in the SA node.
This causes cellular responses.

Question 10

What are the effects of the parasympathetic nervous system on cardiac rate and force?

Answer 10

-ve Chronotropic effect
-ve inotropic effect
-ve dromotropic effect
There may also be arrythmias in the atria.

Question 11

What is the funny current?

Answer 11

This modulates the pacemaker potential. The funny current is a mixed sodium–potassium current that activates upon hyperpolarization and CAMP. It is a depolarising current through Hyperpolarization-activated Cyclic Nuclotide Gated (HCN) channels.

Question 12

What is Ivabradine?

Answer 12

This is a selective blocker of HCN channels that is used to slow heart rate in angina. It works because by reducing that heart rate, there is reduced O2 consumption.

Question 13

What is the process of contraction in cardiac muscle?

Answer 13

1. Ventricular action potential triggers the opening of voltage-gated L type Ca2+ channels.
2. There is a Ca2+ influx into the cytoplasm.
3. This then causes Ca2+ release from the sarcoplasmic reticulum by Calcium-Induced Calcium Release.
4. The Ca2+ bind to Troponin C and shifts Tropomysin out of the actin cleft.
5. There is cross bridge formation between actin and myosin resulting in contraction via sliding filament mechanism.

Question 14

What is the process of

relaxation in cardiac muscle?

Answer 14

1. Repolarisation triggers the closure of voltage-gated L type Ca2+ channels.
2. This stops Ca2+ influx into the cytoplasm.
3. Ca2+ efflux occurs by the Na+/Ca2+ exchanger 1.
4. This stops Ca2+ release from the sarcoplasmic reticulum by Calcium-Induced Calcium Release.
5. The Ca2+ bind to Troponin C and shifts Tropomysin out of the actin cleft.
6. There is cross bridge formation between actin and myosin resulting in contraction via sliding filament mechanism.